Grazing Impacts

Background

The management of Eastern Grey Kangaroos for conservation outcomes in the ACT is undertaken according to the Eastern Grey Kangaroo: Controlled Native Species Management Plan and the associated Nature Conservation (Eastern Grey Kangaroo) Conservation Culling Calculator Notification. The management plan is an instrument under the Nature Conservation Act 2014 and is based largely on the ACT Kangaroo Management Plan, although with some updated policies and references to recent scientific literature.

Since 2009, Eastern Grey Kangaroos have been managed in a number of ACT conservation areas by lethal culling. The number of kangaroos to remain in each area is calculated based on a formula derived from local ecological modelling of the relationships between kangaroos, pasture, and climate (which determines pasture growth). The formula was developed such that kangaroo grazing within managed conservation areas would avoid a loss of ground layer herbage mass (grass) below a ‘minimum threshold’ thought to represent the minimum requirements in terms of habitat to maintain viable populations of ground-layer dependent plant and animal species. For grasslands, this ‘conservation density’ target was calculated to be between 0.6 and 1.5 kangaroos per hectare. This was averaged to 1 kangaroo per hectare for management purposes, with further adjustments being applied according to increasing canopy cover in non-grassland areas.

Recent research

Since this original model was developed, a large amount of local research has been conducted to further quantify the relationships between kangaroos and the grassy ecosystems in which they live. For example, research undertaken within Mulligans Flat and Goorooyarroo Nature Reserves demonstrated that reduced kangaroo grazing increased native biomass and benefited beetle abundance and richness and reptile abundance. Additional research undertaken within the ACT and surrounds has further demonstrated the negative relationship between kangaroo density and biomass, and the related effects on bird, plant and reptile diversity.

Concurrent to this research, the Conservation Research unit of the ACT Government undertook a four year research project to further develop an understanding of the relationships between kangaroo density, kangaroo grazing pressure (‘off-take’), ground layer pasture structure and the diversity of plant and reptile species (as a surrogate for biodiversity more generally) inhabiting local grassy ecosystems. The relationships between each of these four elements (see red arrows below) were quantified statistically using data collected from a range of conservation areas in and around the ACT.

The outcomes of this research demonstrated that grazing by kangaroos is strongly influenced by kangaroo preference, with kangaroos eating more when grass was actively growing and eating less when grass was very long (and thus was less likely to have high availability of fresh ‘green pick’). These effects, along with a weaker effect of kangaroo densityin some native grasslands, were the key factors determining rates of pasture ‘off-take’ for all grass types measured. Results provide useful insight into how and where kangaroo grazing is likely to impact on ground layer habitat structure.

This research project also demonstrated that grass species composition is the key determinant of ground layer structure (e.g. the height and variability of the grassy layer), notwithstanding the previously described relationships between ground layer herbage mass and the density of native grazers. For example, our study found that grass was likely to be tall and homogeneous (i.e. have little variability) in areas dominated by exotic pasture grasses, which are not heavily impacted by kangaroo grazing. Areas of grassy ecosystems dominated by native grasses tended to naturally show a more variable structure (represented by greater variability in grass heights, and the presence of patches of bare ground), especially in open woodlands or other areas of lower average grass height.

A variable grassy structure was also associated with benefits in terms of overall biodiversity indicies (e.g. floristic richness and reptile diversity), in keeping with the current management strategy for the ACT’s grassland ecosystems. This result reflects the value in having multiple different habitat types available to support a range of individual species (e.g. a mix of long, dense grass; medium, patchy grass; and areas of short, open grassy structure), given that each species will have specific preferences across the range of possible ground layer structures. How different areas of the conservation estate are managed to achieve a particular ground layer structure will reflect our understanding of the habitat preferences of resident species of interest (e.g. threatened plants or animals), as well as aiming to provide a broadly appropriate habitat structure to support ecosystem function more generally.

Future directions

The strong influence of the amount of grass, and how quickly it is growing, on the grazing pressure exhibited by kangaroos demonstrates the need for a more dynamic model to be developed for establishing the most appropriate density of kangaroos for a given conservation area under anticipated climatic (i.e. grass growth) conditions. At present, adjustments are made to the calculated number of kangaroos to remain within each reserve based on judgements made by an experienced ecologist. With the development of a more complex model, this process could possibly be formalised based on data collected throughout this research project.

Further research is also warranted into methods of managing habitat structure in patches of the conservation estate avoided by kangaroos (e.g. long, rank grass). A Grasslands Restoration project, jointly funded by the ACT and Commonwealth Governments, is assessing the ability of livestock grazing and ecological burns to ‘re-set’ these degraded areas and increase the condition of the grasslands as both habitat for native species and grazing areas for kangaroos.